BEIJING (AP) — Twenty-one people running a mountain ultramarathon have died in northwestern China after hail, freezing rain and gale-force winds hit the high-altitude race, state media reported Sunday.

After an all-night rescue operation in freezing temperatures involving more than 700 personnel, rescuers were able to confirm that 151 people were safe, out of a total of 172 participants. Twenty-one had died, according to the official Xinhua News Agency, which said the runners suffered from physical discomfort and the sudden drop in temperature.

The runners were racing on an extremely narrow mountain path at an altitude reaching 2,000-3,000 meters (6,500-9,800 feet). The 100-kilometer (60-mile) race was held Saturday in the Yellow River Stone Forest tourist site in Baiyin city in Gansu province.

Participants were not rookies. One of the deceased was a well-known runner Liang Jing, who had won a 100-kilometer (62-mile) race in Ningbo, reported the Paper, a state-backed newspaper based in Shanghai.

The race also followed a relatively established course, having been held four times, according to an account posted online by a participant in the race who quit and managed to make his way to safety.

But the weather caught them off guard, and on the morning of the race Saturday, he already sensed things were not normal. The runners were not dressed for winter-like conditions, many wearing short-sleeved tops.

“I ran 2 kilometers before the starting gun fired to warm up … but the troublesome thing was, after running these 2 kilometers, my body still had not heated up,” the competitor said in a first-person account that has been viewed more than 100,000 times on his WeChat account “Wandering about the South.”

He later told the Paper that the forecast the day prior to the race did not predict the extreme weather they encountered.

The most difficult section, from kilometer 24 (mile 15) to kilometer 36 (mile 22), climbed 1,000 meters (3,280 feet). There, he said the path was just a mix of stones and sand, and his fingers grew numb from the cold.

When he finally decided to turn back, he already felt dazed. He said he was able to make it to safety and met a rescue crew. He did not respond to a request for comment left on his social media account.

Some runners farther along the course had fallen off the trail into deep mountain crevices, according to a reporter for state broadcaster CCTV. It was not clear how many of them survived.

Video footage showed rescuers in winter jackets in the pitch-dark night searching with flashlights along steep hills and narrow paths. Search operations ended by noon Sunday, rescuers told Xinhua.

Online, some wondered what, if any preparations organizers had made in the event of an emergency. The race organizer did not immediately respond to calls seeking comment Sunday.

Baiyin city Mayor Zhang Xuchen held a news conference later Sunday and profoundly apologized as the organizer of the event. The government promised a full investigation.

“We express deep condolences and sympathy to the families of the victims and the injured,” the mayor said.

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Associated Press writer Huizhong Wu in Taipei, Taiwan and news researcher Henry Hou in Beijing contributed to this report.